Co-occurrence and interactions of large invertebrate predators in relation to the Bythotrephes invasio

Since the Bythotrephes invasion in North American Lakes, a number of cladoceran zooplankton species have declined dramatically and overall cladoceran abundance is reduced. In this study I examined Bythotrephes impacts on the native invertebrate predators, including the cladoceran Leptodora kindtii, the glacial relict crustacean, Mysis relicta, and the dipteran larva, Chaoborus spp. These species rely on zooplankton as prey and therefore may suffer as a result of the Bythotrephes invasion. In this study I also examined the role of invertebrate predators in lake food webs in relation to the Bythotrephes invasion. I measured Bythotrephes and native invertebrate abundances, consumption and estimated trophic levels using stable isotopes delta15N and delta13C in a comparison study between inland lakes with Bythotrephes and with no Bythotrephes. In lakes with Bythotrephes the overall consumption demand by invertebrate predators was 25% greater than in lakes with no Bythotrephes. This increase in demand was due to the energetic demands by Bythotrephes although there were lower abundances of both Leptodora and Chaoborus in lakes with Bythotrephes than in lakes without Bythotrephes. Trophic position of native invertebrate predators did not differ between invaded and non-invaded lakes suggesting native invertebrate predators do not feed extensively on Bythotrephes . Stable isotope delta15N and delta13C analysis suggested that there is a greater degree of similarity between the diets of Mysis and Chaoborus and between the diets of Leptodora and Bythotrephes. In addition, there is a high degree of overlap in the vertical distribution of Leptodora and Bythotrephes. Leptodora were smaller and less abundant in invaded lakes than in non-invaded lakes. One possible explanation is competition for prey between Bythotrephes and Leptodora. Indeed cladocerans were smaller in the epilimnion of invaded compared to non-invaded lakes. Understanding the effects of Bythotrephes on native food webs is an ongoing challenge. In this study I demonstrate that Bythotrephes increases the role of invertebrate predators, alters the availability of prey to native invertebrate predators and causes declines in the abundance of Leptodora..